1. Field of the Invention
This invention relates generally to metals processing, and more particularly to wires and methods of making the same.
2. Description of the Related Art
MP35N wires and MP35N-clad-silver wires are used in myriad of applications in industry. One of the most common uses of such wires is in the medical devices area. MP35N has been a material of choice for many years for lead wires in cardiac pacing devices, cardiac rhythm management devices, implantable defibrillators, neuro-stimulation devices, and other similar devices. These wires carry the electrical current from the pulse generator to the electrodes. They are typically either coiled or twisted into cables to produce a highly flexible, fatigue-resistant structure. They are also commonly coated with a polymer for electrical insulation either before or after coiling or cabling.
The wire for these medical devices should exhibit biocompatibility and be very tough and fatigue resistant. Historically, toughness has been specified only by requiring a certain tensile elongation in the final wire.
As with most metal wire production, conventionally produced MP35N clad silver wire goes through a series of cold draws with anneals interspersed. The cold draws bring the wire to a desired outer diameter and tensile strength. The anneals serve the purpose of softening the metal to allow further cold reduction. The amount of cold work and the annealing parameters (time and temperature) are tailored to obtain the final desired strength and perhaps other properties. For MP35N clad silver wire, the annealing temperature is typically around 1700° F. which is below the approximate 1761° F. melting point of silver, and well below the 1900 to 2000° F. temperature range at which MP35N is typically annealed. A final anneal is usually performed before a short series of final draws. The final anneal in conventional processing is short, on the order of twenty seconds or less. Such short anneals can provide the wire with relatively high final ultimate tensile strength and sufficient ductility to undergo the final draws.
A difficulty associated with conventionally produced MP35N clad silver wire is the potential for relatively low torsional strength. The problem stems from the fact that wire designers and consumers place great emphasis on ultimate tensile strength. Reliance on ultimate tensile strength as a measure of wire behavior is not necessarily counterintuitive. Knowledge of ultimate tensile strength provides a familiar measure of a material's ability to withstand commonly applied stresses, such as shear, bending, tension and compression. However, for structures subjected to torsion, such as coiled wires in a medical device lead, ultimate tensile strength alone may not be enough to predict the behavior of the wire.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.